1. Field of Invention
The present invention relates to novel methods of forming electrodes suitable for use in an electrochemical cell. The invention also relates to novel electrodes which can be formed by such methods. The invention further relates to electrochemical cells which include the inventive electrodes. The invention has particular applicability to the manufacture of nonaqueous electrochemical cells.
2. Description of the Related Art
Electrochemical cells, for example, nonaqueous power supplies are formed from various components such as electrodes, separators and seals. These components are typically formed from finely divided particulate materials, the particles of which must be bonded together to form cohesive structures which in turn may also be bonded to other structural element(s) in the power supply. This function of bonding materials together is conventionally accomplished by the use of monomers, polymers, copolymers or mixtures thereof, collectively referred to as binders for these applications.
Binders in electrochemical cells should be insoluble in the electrolyte and chemically inert with respect to the other materials in the cell. The binders should not be oxidized or reduced within the cell, and the binders should be thermally stable during storage and over the operating temperature range of the cell. The binder should also have sufficient mechanical integrity and flexibility to bond the appropriate materials of the cell and to prevent cracking, spalling, exfoliation and other forms of mechanical disintegration caused by thermal expansion/contraction or by the expansion/contraction which accompanies the charge/discharge cycling of the cell.
A commonly used polymer binder is polytetrafluoroethylene (PTFE), sold under the tradename TEFLON.RTM.. This material is chemically inert and does not dissolve in electrolytes or solvents. Because of the chemical inertness of this binder, electrodes are difficult and laborious to fabricate, and adhesion to current collectors is poor. Typically, thick electrodes must be made by blending aqueous dispersions of PTFE with active electrode ingredients and conductive additives. The slurry formed is then drawn, shaped and kneaded to the desired consistency. It is then formed into an electrode and baked at high temperature to remove the water. The electrode is then pressed into current collector grids. PTFE flows plastically under compression and it is lyophobic to most electrolytes. This material is not suitable for advanced high power cells with thin electrodes on foil current collectors.
Polyethylene is also widely used in nonaqueous electrochemical cells. See U.S. Pat. No. 4,731,310, to Anderman et al. This material is commonly used as a binder for anode and cathode composites, and it is a major ingredient in some separators and seals. A copolymer consisting of ethylene and chlorotrifluoroethylene (ECTFE), commercially available under the tradename HALAR.RTM., has also been used as a binder in nonaqueous cells. See U.S. Pat. No. 4,228,228, to Beauchamp. Unlike PTFE, polyethylene and chlorotrifluoroethylene components can be melted and hot blended together with powdered components. Electrodes or separators are typically hot pressed or extruded to shape, which processes avoid the use of solvents. The hot viscous pastes are not, however, suited to the manufacture of thin electrodes on thin foil current collectors, and adhesion to most surfaces is relatively poor.
Improved manufacturability of thin electrodes has been achieved by the use of soluble fluorinated homopolymers such as polyvinylidene fluoride (PVDF) or copolymers of vinylidene fluoride and hexafluoropropylene (VF2-HFP), or of vinylidene fluoride and chlorotrifluoroethylene (VF2-CTFE). See U.S. Pat. Nos. 5,571,634 and 5,587,253, to Gozdz et al. These homopolymers or copolymers are typically dissolved in a solvent such as N-methyl pyrrolidone, and then blended into a slurry with the active electrode material and a carbon conductive additive. The slurry is coated onto a foil current collector, and the solvent is evaporated to yield an electrode structure. While this process can yield thin electrodes, adhesion of the electrode to the current collector is still poor and often requires the use of adhesion promoters. Furthermore, the fluorinated polymers are partially soluble in the alkene carbonate electrolytes which are commonly used in electrochemical cells. The fluorinated polymers also form gels and swell in the presence of alkylene carbonate electrolytes.
Improvement in adhesion and solvent resistance has been achieved by the copolymerization of vinylidene fluoride monomers with (a) epoxy groups containing vinyl monomers and (b) unsaturated dibasic acid monoester, as disclosed in U.S. Pat. No. 5,739,234, to Kashio et al.
U.S. Pat. No. 5,436,093, to Huang et al, discloses that ethylene propylene diene monomer (EPDM) can be used as a binder for carbon anode electrodes in lithium-ion batteries. These electrodes are coated to thicknesses of 10 to 15 mil (254 to 381 .mu.m).
U.S. Pat. Nos. 4,810,755, to Akazawa et al, and 4,857,600, to Gross et al, disclose that polymers containing grafted maleic acid or maleic anhydride have improved adherence to metal foils such as aluminum foil used in food wraps. These documents, however, disclose the use only of nonchlorinated polymers.
U.S. Pat. No. 5,464,707, to Moulton et al, discloses that polymers containing grafted acid functionalities can be used in electrically conductive adhesion promoters in electrochemical cells. Specifically, maleic anhydride can be grafted onto a polymer and subsequent hydrolysis of the maleic anhydride will provide for the adhesion promoting carboxylic acid functionality on the polymer. This document, however, only discloses nonchlorinated polymers. Furthermore, polymers that contain grafted acid functionalities are disclosed only as binders in adhesion promoting films, and not as binders in anode, cathode or separator elements.
To meet the requirements of the electrochemical cell manufacturing industry, and to overcome the disadvantages of to the related art, it is an object of the present invention to provide novel methods of forming electrodes suitable for use in an electrochemical cell. Through the invention, it has been found that hardenable coating compositions previously formulated for protecting and/or beautifying polypropylene resins can effectively be employed as a binder in nonaqueous electrochemical cells. The coating composition of the invention is readily soluble in selected solvents, so that anode and cathode slurries or inks can be formulated and coated or printed onto current collector substrates. The resulting anode or cathode structures have improved performance when compared with electrodes of the related art.
It is a further object of the invention to provide novel electrodes which can be formed by the inventive methods.
It is yet a further object of the invention to provide novel electrochemical cells which include the inventive electrodes.
Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art on a review of the specification, drawings and claims appended hereto.